1. Field of the Invention
The present invention relates to a radio communication system for providing communication services to mobile terminal devices and a communication channel allocation method and a communication control device in that system.
2. Description of the Related Art
In the simplified portable telephone known as the so called PHS (Personal Handyphone System), the mobile terminal device has a smaller output (10 mW) compared with the current digital cellular phone system so that a range over which the mobile terminal device can carry out communications with the base station has been limited. Also, the scheme used in the existing PHS is capable of carrying out the data communications that are faster (32 KBPS to 64 KBPS, for example) compared with the current digital cellular phone system.
On the other hand, W-CDMA (Wideband-Code division Multiple Access) has been proposed as a scheme for realizing fast data communications (64 KBPS to 384 KBPS, for example).
This W-CDMA uses radio signals of 2G band (uplink: 1.92 to 1.98 GHz, downlink: 2.11 to 2.17 GHz), for example. Also, the PHS described above uses radio signals of 1.9 GHz band (12.89365 to 1.91945 GHz).
As such, these radio communication systems are using close frequencies so that, in order to avoid interferences between them, the so called guard band is provided between the frequency bands to be used by the respective radio communication systems. For example, the guard band of about 5 MHz is provided between the upper limit frequency of the frequency band to be used by the PHS and the lower limit frequency of the frequency band to be used by the W-CDMA.
Also, the devices to be used by the respective radio communication systems use a filter for reducing components outside a prescribed band (channel width), so as to avoid interferences between these radio communication systems.
However, depending on the positional relationships among the mobile terminal devices and the base stations of both the PHS and the W-CDMA or the like, there are cases where the components outside a prescribed band can be reduced sufficiently on one radio communication system but cannot be reduced sufficiently on another radio communication system.
In particular, the signal band of the W-CDMA is about 5 MHz, for example, so that the so called spurious components or noises are generated over a wide frequency region that may be extended beyond the guard band such that they can potentially cause interferences to the PHS side.
Also, the transmission power of the W-CDMA is considerably larger than the transmission power of the PHS so that there can also be cases where the so called receiver blocking occurs at the receiver of the PHS side to lower the reception sensitivity.
In order to avoid interferences with respect to the PHS, it is possible to consider the suppression of the generation of the spurious components or noises outside a prescribed band by improving the performance of filters on the W-CDMA side. However, a difference in the transmission power between the PHS and the W-CDMA is so large that an extremely severe blocking characteristic (the attenuation rate in the blocking region) would be required to the filters on the W-CDMA side if the components outside a prescribed band are to be suppressed to the level of not influencing the PHS side under any circumstances. The filter for realizing such a characteristic would have a complicated structure using many elements, which would require large power consumption and size. For this reason, the use of such a filter is difficult especially in the mobile terminal device for which demands for a smaller size and a lower power consumption are high.
It is also possible to consider the provision of providing a filter on the receiver of the PHS in order to prevent mixing of noises from the W-CDMA, but for the similar reason, this provision would make the device complicated and increase its cost.